In C++, quantifiers are special symbols that are used in regular expressions to specify the number of times a character, group of characters, or pattern should occur. There are several quantifiers available in C++, including the following:

1. *: The * quantifier specifies that the preceding character, group of characters, or pattern should occur zero or more times. For example, a* matches zero or more a characters.
2. +: The + quantifier specifies that the preceding character, group of characters, or pattern should occur one or more times. For example, a+ matches one or more a characters.
3. ?: The ? quantifier specifies that the preceding character, group of characters, or pattern should occur zero or one time. For example, a? matches zero or one a characters.
4. {n}: The {n} quantifier specifies that the preceding character, group of characters, or pattern should occur exactly n times. For example, a{3} matches exactly three a characters.
5. {n,}: The {n,} quantifier specifies that the preceding character, group of characters, or pattern should occur at least n times. For example, a{3,} matches at least three a characters.
6. {n,m}: The {n,m} quantifier specifies that the preceding character, group of characters, or pattern should occur at least n times, but no more than m times. For example, a{3,5} matches at least three a characters, but no more than five a characters.

Here is an example of how to use quantifiers in a regular expression in C++:

#include <iostream>
#include <regex>

int main()
{
std::string s = "aaaabbbbccccdddd";
std::regex r("a*b+c{2,4}d{2}");
if (std::regex_match(s, r))
{
std::cout << "Match found!" << std::endl;
}
else
{
std::cout << "Match not found." << std::endl;
}
return 0;
}

In this example, the regular expression a*b+c{2,4}d{2} uses all of the quantifiers listed above. It matches a string that contains zero or more a characters, one or more b characters, two to four c characters, and exactly two d characters. In this case, the string "aaaabbbbccccdddd" matches the regular expression, so the output is “Match found!”.